There are prior art patents that describe composite seals that are fabricated of resilient material that encapsulates or is adjacent to a metal seal ring. For example, U.S. Pat. Nos. 4,344,629 and 4,394,020 describe and teach mechanical metal-to-metal seals, the metal sealing surfaces of which are not pre-lapped prior to assembly. Both of these patents feature a resilient seal that initially operates to provide a seal while the metal-to-metal dynamic sealing surfaces are being lapped-in during operation of the rotary cone rock bit in a bore hole. U.S. Pat. No. 4,344,629 teaches a non-lapped metal ring that is encapsulated within a resilient material. The resilient material serves to initially provide a seal while the metal surfaces progressively lap themselves in as each rotary cone rotates on its journal while the bit works in a bore hole. U.S. Pat. No. 4,394,020 teaches a metal ring that is bonded to resilient material with a separate O-ring confined within an annular slot formed in the metal ring of the seal. The O-ring, like that in U.S. Pat. No. 4,344,629 serves to provide the initial seal while the dynamic metal sealing surfaces are being lapped-in during bit operation.
U.S. Pat. No. 4,666,001 describes an earth-boring bit with metal-to-metal seals. A rigid face seal assembly is positioned between the cutter and bearing shaft of the rotary bit. The seal assembly moves axially in response to and to compensate for dynamic pressure changes in the lubricant adjacent to the seal. The mechanical metal-to-metal seal is designed to translate axially within an enlarged seal cavity during operation of the bit in a bore hole to react to sudden changes of pressure across the seal face.
The foregoing patent is similar to a typical "Caterpillar" type seal, examples of which are found in U.S. Pat. Nos. 3,180,648, 3,403,916, and 3,524,654 all of which are assigned to the Caterpillar Corporation and which teach improvements to their basic metal-to-metal seal.
These patents, while they illustrate and teach mechanical metal-to-metal seals, do not elaborate on the materials used to form the mechanical sealing surfaces. U.S. Pat. No. 4,824,123 describes and teaches a metal-to-metal seal comprising a static metal seal ring and a dynamic metal seal ring, the sealing surface of the static seal ring comprising a relatively softer metallic material than an opposite dynamic sealing surface. The use of opposing metallic sealing surfaces each having a different metallic hardness was believed to be beneficial as the relatively softer sealing material would be expected to wear at a faster rate to accommodate for cone wobble or eccentricities that may occur between the cone and the journal during operation of the rotary cone rock bit in a bore hole. Accordingly, the relatively softer sealing surface was in effect sacrificial in nature.
It has been discovered that the use of a mechanical seal comprising the pairing of a seal ring made from a soft material with a seal ring made from a hard material, although serving to accommodate cone wobble or eccentricities that may occur during use, also undergoes accelerated wear during such use that ultimately reduces the service life of the rock bit. As the sacrificial mechanical seal member begins to wear, the spatial tolerance between the rotating cone and the rock bit journal increases. With continued mechanical seal wear, the tolerance increases to a point where either the lubricant kept within the cone to lubricate the cone bearings is allowed to escape past the seal, or abrasive drilling mud directed near the rock bit is allowed to enter into the cone past the seal. The occurrence of either event generally has the effect of decreasing the service life of the rock bit.
It is, therefore, desirable that a rotary cone rock bit comprise a mechanical seal having sealing materials constructed in a manner that reduces the potential of premature wear or failure of the sealing surfaces during service at the high temperatures, bearing pressures and rotational speeds often found in modern rock bits, thus enhancing the service life of such rock bits. It is also desirable that the mechanical seal promote optimum sealing during the use of the rock bit and that the mechanical seal have sealing members made from materials that are both relatively available and economic to construct using practical manufacturing methods.